In many electronic control systems, a power source relay is used to control a power supply to an electronic controller, actuators and the like. The power source relay may sometimes be held fixed to any one of the ON-state and the OFF-state contrary to the instruction from an external instruction signal to change the ON/OFF state. For instance, a movable contact is held inseparably fused to a fixed contact, and is uncontrollable to the other state. It is necessary to detect fusion (welding) of the power source relay. One example of such a fusion detecting circuit is disclosed in JP 2004-32903A.
One method for detecting a fusion is proposed as shown in FIG. 8. Specifically, the input side of a power source relay 33 is connected to a power source such as a battery 8 or the like, and the output side thereof is connected to a target such as a solenoid driving circuit 32 in an electronic controller 3 for a solenoid 2 in an actuator 12. The actuator 12 may be a transfer-ratio varying mechanism for a vehicle steering system, which is to be supplied with electric power. A capacitor C205 and a resistor R204 are connected to the output side of the power source relay 33 for detecting the fusion of the power source relay 33.
When the power source relay 33 is set to the ON-state to start the operation of the solenoid 2, the voltage of the battery 8 is applied between both ends of the capacitor C205. The charge corresponding to the voltage PIG in FIG. 9 is accumulated.
Under this condition, a duty ratio of a PWM signal is transmitted as a control instruction to the solenoid 2 from the microcomputer 31. The solenoid 2 is connected in parallel with a diode D522. A transistor T511 is subjected to duty operation through a resistor R539 and a diode D535 to drive the solenoid 2. By changing the duty ratio of the PWM signal from the microcomputer 31, the current flowing in the solenoid 2 is varied, and the electromagnetic force generated by the solenoid 2 is also varied.
When the power source relay 33 is set to the OFF-state to stop the operation of the solenoid 2, the capacitor C205 starts to discharge the charge accumulated therein. The charge thus discharged (that is, current) is consumed by the resistor R204. The voltage of the capacitor C205, that is, the output voltage of the power source relay 33, normally decreases with time as shown in FIG. 9.
The power source relay 33 is detected as having been fused, if an output voltage VIG of the relay 33 (i.e., the terminal voltage of the capacitor C205) does not become lower than a threshold voltage value Vth set for the fusion detection within a predetermined detection time T1 after the power source relay 33 is set from the ON-state to the OFF-state in response to an instruction from an external side such as a microcomputer 31. This fusion detection may be attained by the microcomputer 31 or by other circuits.
In the fusion detecting method for the power source relay described above, it is desired that the detection time T1 is short. Furthermore, the voltage VIG can be represented by PIG X exp(-t/RC). Accordingly, it is desired that the value of the resistor R204 for discharging the charge accumulated in the capacitor C205 is small. However, current flows in the resistor R204 during normal operation, that is, when the power source relay 33 is kept set to the ON-state. Thus, if the value of the resistor R204 is small, the current value is increased. The current flowing in the resistor R204 is not used to drive the solenoid 2 and is loss current.
This loss current is consumed by the resistor R204, so that the resistor R204 is heated. When the heating amount of the resistor R204 is increased, the ambient temperature of the surrounding of the resistor R204 and the temperature of the substrate rise up. As a result, electronic circuit parts durable to the rise-up of the temperature must be used. This is, the parts to be used are restricted, resulting in increase of the part cost and the manufacturing cost.